Feature #6497: Disabling TLS client-side renegotation

• Status: Closed
• Assignee: MartinBosslet (Martin Bosslet)
• Target version: 2.0.0

[MartinBosslet (Martin Bosslet)]

I added support for completely disabling client renegotiation on SSL/TLS servers in r35797.
Client renegotiation is still considered a problem, even with secure renegotiation support.

It's now possible to either completely disable client renegotiation at all or to specify
a maximum number of handshakes. The feature is opt-in, the default is as it was before,
to allow arbitrary client renegotiation attempts. The feature is meant to help in
scenarios where the OpenSSL extension is used to run a server that should not support
client renegotiation for security reasons.

Because we don't support renegotiation in the OpenSSL extension, it wasn't possible
to write explicit test cases, but I created a simple server script [1] that can be
tested with tools such as OpenSSL's s_client [2]. For example, when running the
server script at [1], testing the feature could be accomplished by:

openssl s_client -connect localhost:8443

And then pressing 'R' and Enter, you should receive an error like this:

RENEGOTIATING
140639302223680:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake failure:s3_pkt.c:591:

If you don't, any feedback is much appreciated!

[1] https://gist.github.com/2791400
[2] http://blog.ivanristic.com/2009/12/testing-for-ssl-renegotiation.html

[MartinBosslet (Martin Bosslet)]

Updated in r35994. There is a generic renegotiation_cb attribute for SSLContext now instead of explicit configuration parameters. This reduces the code in ossl_ssl.c and gives users maximum flexibility on how they'd like to act upon renegotiation attempts.

A simple "disable client renegotiation entirely" callback could be implemented as follows:

num_handshakes = 0
ctx.renegotiation_cb = lambda do |ssl|
num_handshakes += 1
raise RuntimeError.new("Client renegotiation disabled") if num_handshakes > 1
end

This way, the initial handshake passes, but further attempts will be rejected. I also updated
the test server script at https://gist.github.com/2791400.

[MartinBosslet (Martin Bosslet)]

• Status changed from Feedback to Closed

Closing, as it did not seem to have negative impact of any kind so far.